Field of the Invention
The invention concerns a method to determine a magnetic resonance image from magnetic resonance data entered into k-space that are acquired with a magnetic resonance apparatus, wherein, in the acquisition, a deviation (described by an interference field) from ideal homogeneity is present in the imaging region covered by the magnetic resonance data. The invention concerns a magnetic resonance apparatus to implement such a method.
Description of the Prior Art
In magnetic resonance imaging (often also called magnetic resonance tomography), a basic field magnet of a magnetic resonance apparatus generates a homogeneous basic magnetic field (most often designated as B0). For example, the basic magnetic field can have a strength of 1.5 or 3 T. In a defined region (known as the homogeneity region) around the isocenter of the basic magnet, accuracies with deviations in the ppm range are achieved. To excite nuclear spins in a subject for magnetic resonance imaging, the basic magnetic field is superimposed with an excitation magnetic field (known as the B1 field) aligned orthogonal to the basic magnetic field that is weaker by five to six times orders of magnitude than the basic magnetic field. The B1 field or excitation magnetic field is generated by radio-frequency coils based on an excitation pulse that is part of the magnetic resonance sequence to acquire magnetic resonance data. Additionally, in magnetic resonance imaging magnetic field gradients are used that have a linear curve in the homogeneity region of the basic magnetic field and that are smaller by two to three orders of magnitude than the basic magnetic field. With the use of such gradient fields, the frequency and the phase of the signals produced by the excited nuclear spins are coded so that ultimately a path in k-space is defined along which the magnetic resonance data are entered into k-space. The magnetic resonance data in k-space must then be transferred into the image domain (i.e., image data that can be displayed).
It has long been the state of the art to apply a Fourier transformation in order to transform the magnetic resonance data from k-space into the image domain, and thus to obtain magnetic resonance images that show the imaging region in spatial coordinates.
In practice, the homogeneity of the basic magnetic field in the homogeneity region and the linearity of the gradient fields can be negatively affected by interference fields. For example, interference fields can be created by imperfections in the gradient system or by induced magnetic fields in the examined subject. Implants, dental braces and the like in the patient as a subject to be examined can be responsible for this. These interference fields generally lead to artifacts in the magnetic resonance image (known as susceptibility artifacts) since the frequency coding and phase coding of the spins is disrupted and signals are either lost due to dephasings or are rendered with distortion in the image. Due to the interference field, it may occur that spins in a curved slice of the subject are excited, rather than a rectilinear slice as intended. Artifacts also arise in this manner outside of the homogeneity region, wherein the fields do not have a linear or homogeneous curve.
For metal-induced artifacts in the overall problem in magnetic resonance imaging is described in summary in an article by Brian A. Hargreaves at al., “Metal-Induced Artifacts in MRI”, AJR:197, Page 547-555, September 2011.
Various approaches are known in order to keep the influence of the interference fields on the magnetic resonance image as small as possible. As described in the cited article, in one approach the dephasing and distortion during the data acquisition can be kept as minimal as possible by suitable sequence parameters, such as high readout pulse bandwidth. In another approach, after the Fourier transformation the distortion can be corrected in the image domain. Correction in the image domain is only reasonable for small disruptions since, due to the superposition of multiple image points a calculation can no longer be made as to which signal intensity originates from which original image point. Variations in the measurement protocol or in the magnetic resonance sequence are difficult to reconcile and must be made for each subject to be acquired.